1. Field of the Invention
The present invention relates to an exhaust device for use in an internal combustion engine and, more particularly, to an exhaust device for a multiple cylinder engine in which an independent exhaust passage is provided for each cylinder.
2. Description of the Prior Art
There is well known a turbocharger which is operated by exhaust gases from an engine, and which is a kind of supercharger for increasing the intake pressure to thereby increase the volume of the suction air to be charged into a cylinder of the engine. The turbocharger has in the exhaust passage an exhaust gas turbine which is rotated by the exhaust gases to drive a pump for compressing the air.
In an engine provided with the turbocharger, to improve the charging response during lower speed driving of the engine the independent exhaust passages for respective cylinders should be somewhat of small diameter to decrease the volume of portions thereof upstream of the turbine, which however results in high temperature and high pressure of the exhaust gases during high-speed, high-loaded conditions of the engines. When these high-temperature, high-pressure gases are directly supplied to the exhaust gas turbine, a large thermal load is imposed on the turbine.
On the other hand, also in an engine without a turbocharger, i.e. a natural aspiration engine, the exhaust gases become of high temperature and high pressure during the high-speed, high-loaded engine conditions so that there is the possibility that the exhaust manifold and an exhaust emission control device in the exhaust passage downstream of the manifold may be deteriorated prematurely. An attempt to solve this problem by enlarging the diameter of the exhaust passage would cause a disadvantageously large size of the exhaust device.
In view of this situation it has been considered necessary to set an air-fuel ratio richer than the theoretical air-fuel ratio, so that an excessive amount of fuel which is not burned in the cylinder consumes energy of the exhaust gases when vaporized, resulting in lowering the temperature of the exhaust gases. This solution, however, involves not only poor fuel consumption but also a decrease in torque due to the richer air-fuel ratio.
Japanese Utility Model Laid-Open (Kokai) No. 59-148427 discloses an exhaust manifold having a partition which divides an internal passage of the manifold in such a manner as to avoid interaction of the exhaust gases from cylinders and in which is provided a communication port adapted to open when the number of revolutions of the engine exceeds a predetermined value. This proposed structure might lower the temperature of the exhaust gases to some degree due to a decrease in gas pressure, but the extent of such lowering is still unsatisfactory.
Accordingly, an object of the present invention is to provide an exhaust device for an internal combustion engine which will lower the temperature and pressure of the exhaust gases during high-speed, high-loaded engine conditions without the necessity of setting the air-fuel ratio richer.
Another object of the invention is to provide an exhaust device which can improve fuel consumption without having reduced durability.